Are all the dead BG large (8in.)? Have you seen any small ones ? Are there both male and female dead BG ? Have the smaller (not small) ones been females. Do they show any outward signs of sores or fungus or wounds? As you can tell it is often hard to tell why. Stressors of fish are many and what you see or find is a symptom not the cause. The cause is what you need to find out so as to limit or remove it if possible. See the links below.
If all the dead BG are large adults both male and female and your pond is 6+ years old , given the age of the fish do not rule out the possibility of natural morts. especially if they are under weight as per Bill's comment.
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